1. Field of the Invention
The present invention relates to an anisotropic conductive film that is capable of electrically and physically connecting between an electronic part of IC chip or a liquid crystal panel (LDC panel) of a liquid crystal display (LCD) and a substrate, or between substrates, and also relates to a method for efficiently producing the anisotropic conductive film, and a bonded structure for the electronic part and the substrate, etc. using the anisotropic conductive film.
2. Description of the Related Art
Anisotropic conductive adhesion films (ACF: Anisotropic Conductive Film) have conventionally been used as a means for connecting an electronic part and a circuit substrate. The anisotropic conductive adhesion film is used for adhering and electrically connecting between various terminals, for example a case for connecting a flexible print substrate (FPC) or a terminal of IC chip with ITO (Indium Tin Oxide) electrode formed on a glass substrate of LCD panel.
Commonly used anisotropic conductive adhesion film is a single-layered film in which conductive particles are dispersed in an epoxy resin insulating adhesive layer. For example, terminals of IC chip and ITO electrode of a glass substrate are electrically connected by disposing the conductive particles between the terminals of the IC chip and the ITO electrode.
The recent trends for miniaturized and high performance electronic devices lead to connection terminals of fine pitch, and as a result, the connection area of the terminal is reduced. Even though the connection area is reduced, reliability for high conduction is still required.
However, the single-layered anisotropic conductive adhesion film has a problem such that conductive particles are moved along with the insulation adhesive by the heat and pressure at the time of the connection. To ensure the electrical connection with such the anisotropic conductive adhesion film, it is necessary to maintain at least certain number of conductive particles on the terminal after the connection, and thus the number of the conductive particles contained in the insulating adhesive need to be increased.
In the art, there are also proposed a two-layered anisotropic conductive film. For example, the one having one layer (ACF) in which conductive particles are dispersed in an insulating adhesive and the other layer (NCF: non conductive film) formed of the same insulating adhesive but not dispersing any conductive particles therein, or the one having one layer in which conductive particles are dispersed in an insulating adhesive and the other layer formed of an insulating adhesive part of which is replaced with a different insulating adhesive to the insulating adhesive used in the layer containing the conductive particles. When such the anisotropic conductive adhesion film is used, terminals of IC chip are buried into the layer which does not contain the conductive particles, and the conductive particles dispersed in the other layer are sandwiched between the terminals of the IC chip and ITO electrode to thereby electrically connect the IC chip and ITO electrode. Therefore, the number of the conductive particles moved into the gaps between the terminals are reduced, and the ratio of the conductive particles stayed on the connection terminal (i.e. a particle capturing rate) is improved even though the amount of the conductive particles contained is smaller than that of the single-layered anisotropic conductive adhesion film.
However, pitch between terminals of electronic devices has been getting smaller and smaller as a trend, and if the two-layered anisotropic conductive adhesion film is used in such the electronic device, conductive particles are moved along with an insulating adhesive to the surrounding of IC chip by heat pressure at the time of the connection and the moved into the gap between terminals, causing a short circuit. Moreover, it is also insufficient in terms of the particle capturing rate. Therefore, it has been desired to develop a technology capable of preventing the movement of conductive particles at the time of connection.
Aiming at meeting with such demand, there has been proposed an anisotropic conductive adhesion film in which a photo-curable resin is used as an insulating binder, and conductive particles are dispersed and aligned into a monolayer in the photo-curable resin then exposed to UV light so as to fix the conductive particles in the binder (see Japanese Patent Application Laid-Open (JP-A) No. 2001-52778). As the conductive particles are fixed in the binder, this film prevents the conductive particle from moving at the time of the connection between terminals and an electrode, inhibits an occurrence of a short circuit and also maintains a high particle capturing rate. However, in the anisotropic conductive adhesion film disclosed in JP-A No. 2001-52778, the layer containing conductive particles (i.e. the conductive particle layer) is completely cured. In order to physical bond the IC chip with the glass substrate, it is necessary to have a three-layered structure in which the conductive particle layer is sandwiched with two adhesive insulating resin layers. The production of such the anisotropic conductive adhesion film has more processes than that of two-layered film, and thus the production efficiency is lowered. Moreover, it is difficult to accurately form the conductive particle layer at the thickness of 5 μm or less using a normal roll coater or comma coater.
There has been also proposed a two-layered anisotropic conductive adhesion film containing an insulating adhesive layer and an insulating binder layer in which conductive particles are embedded in the state of a monolayer (see JP-A No. 2003-64324). The insulating adhesive layer disclosed in JP-A No. 2003-64324 is cured by the radiation of actinic light, but is not cured and has adhesiveness before use. Since the layer containing the conductive particles has adhesiveness, this film enables to physically connect between IC chip and a glass substrate even though it has a two-layered structure. However, after these layers are preliminary bonded, this film needs to be subjected to the actinic light radiation so as to fix the conductive particles in the binder. In other words, users need to install a new apparatus for the exposure of the actinic light, and to control the conditions such as heat and light radiation. Accordingly, user's charge is increased at the time of use, and convenience is reduced.
As have been mentioned above, it is the current situation that there has not been provided a two-layered anisotropic conductive film which prevents the movement of conductive particles at the time of the connection between the electronic parts and the substrate or the like so as to prevent the occurrence of a short circuit, maintains high particle capturing rate so as to obtain excellent conductive reliability and is easily used.